Returns a child UL of a WordPress formatted menu

get_nav_menu_part.php

function get_nav_menu_part( $menu_name, $sub_item_to_find ){
		
	if( !is_string($menu_name) ){
		return;
	}
	if( !is_string($sub_item_to_find) && !is_integer($sub_item_to_find) ){
		return;
	}
		
	// Get the page ID we're looking for to return the ID, either by string or ID
	if( is_string( $sub_item_to_find )){
		$page_to_find = get_page_by_title( $sub_item_to_find );
		$page_id = $page_to_find->ID;
		$dom_class_name = "page-item-$page_id";
	}
	if( is_integer($sub_item_to_find) ){
		$dom_class_name = "page-item-$sub_item_to_find";
	}
		
	// Get main menu as HTML
	$menu_html = wp_nav_menu( 
		array(
			'menu' => $menu_name,
			'echo' => false
		)
	);
		
	// Create a DOM element, and push our menu HTML into it
	$dom = new DOMDocument();
	$dom->loadHTML($menu_html);
		
	// find: li[class='$dom_class_name'] ul
	$finder = new DomXPath($dom);
	$found_marching_nodes = $finder->query("//li[contains(@class, '$dom_class_name')]/ul");
		
	// No nodes? Return the list main menu
	if( !$found_marching_nodes ){
		return $menu_html;
	}
		
	// Push the found nodes into a new DOM, then return the HTML
	$child_dom = new DOMDocument();
	foreach( $found_marching_nodes as $node ){
		$child_dom->appendChild($child_dom->importNode($node,true));
	}
	$output = $child_dom->saveHTML();
	return $output;
		
}
	
	
// Usage:
get_nav_menu_part('Main Navigation', 'Our Services')

// FFS - Or use: http://wordpress.stackexchange.com/questions/2802/display-a-portion-branch-of-the-menu-tree-using-wp-nav-menu